From f7e508423e715c53faf04ed6593f936cb6ceac82 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sat, 12 Jun 2021 17:51:08 +0200
Subject: [PATCH 01/18] Allow plotting categorical data

---
 xarray/plot/plot.py  | 16 ++++++++++++----
 xarray/plot/utils.py |  9 ++++++++-
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index f530427562a..94ab90ebda1 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -1011,9 +1011,13 @@ def pcolormesh(x, y, z, ax, infer_intervals=None, **kwargs):
         else:
             infer_intervals = True
 
-    if infer_intervals and (
-        (np.shape(x)[0] == np.shape(z)[1])
-        or ((x.ndim > 1) and (np.shape(x)[1] == np.shape(z)[1]))
+    if (
+        infer_intervals
+        and not np.issubdtype(x.dtype, str)
+        and (
+            (np.shape(x)[0] == np.shape(z)[1])
+            or ((x.ndim > 1) and (np.shape(x)[1] == np.shape(z)[1]))
+        )
     ):
         if len(x.shape) == 1:
             x = _infer_interval_breaks(x, check_monotonic=True)
@@ -1022,7 +1026,11 @@ def pcolormesh(x, y, z, ax, infer_intervals=None, **kwargs):
             x = _infer_interval_breaks(x, axis=1)
             x = _infer_interval_breaks(x, axis=0)
 
-    if infer_intervals and (np.shape(y)[0] == np.shape(z)[0]):
+    if (
+        infer_intervals
+        and not np.issubdtype(y.dtype, str)
+        and (np.shape(y)[0] == np.shape(z)[0])
+    ):
         if len(y.shape) == 1:
             y = _infer_interval_breaks(y, check_monotonic=True)
         else:
diff --git a/xarray/plot/utils.py b/xarray/plot/utils.py
index 416d56aa620..db85a5908c0 100644
--- a/xarray/plot/utils.py
+++ b/xarray/plot/utils.py
@@ -604,7 +604,14 @@ def _ensure_plottable(*args):
     Raise exception if there is anything in args that can't be plotted on an
     axis by matplotlib.
     """
-    numpy_types = [np.floating, np.integer, np.timedelta64, np.datetime64, np.bool_]
+    numpy_types = [
+        np.floating,
+        np.integer,
+        np.timedelta64,
+        np.datetime64,
+        np.bool_,
+        np.str_,
+    ]
     other_types = [datetime]
     try:
         import cftime

From 0a6cce93f9a602348f761100acd7d018fdd85c2d Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sat, 12 Jun 2021 18:08:58 +0200
Subject: [PATCH 02/18] change tests

---
 xarray/tests/test_plot.py | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/xarray/tests/test_plot.py b/xarray/tests/test_plot.py
index bbd7c31fa16..afbd228b9aa 100644
--- a/xarray/tests/test_plot.py
+++ b/xarray/tests/test_plot.py
@@ -684,10 +684,9 @@ def test_format_string(self):
     def test_can_pass_in_axis(self):
         self.pass_in_axis(self.darray.plot.line)
 
-    def test_nonnumeric_index_raises_typeerror(self):
+    def test_nonnumeric_index(self):
         a = DataArray([1, 2, 3], {"letter": ["a", "b", "c"]}, dims="letter")
-        with pytest.raises(TypeError, match=r"[Pp]lot"):
-            a.plot.line()
+        a.plot.line()
 
     def test_primitive_returned(self):
         p = self.darray.plot.line()
@@ -1162,10 +1161,9 @@ def test_3d_raises_valueerror(self):
         with pytest.raises(ValueError, match=r"DataArray must be 2d"):
             self.plotfunc(a)
 
-    def test_nonnumeric_index_raises_typeerror(self):
+    def test_nonnumeric_index(self):
         a = DataArray(easy_array((3, 2)), coords=[["a", "b", "c"], ["d", "e"]])
-        with pytest.raises(TypeError, match=r"[Pp]lot"):
-            self.plotfunc(a)
+        self.plotfunc(a)
 
     def test_multiindex_raises_typeerror(self):
         a = DataArray(

From 2ccfa6bd735a2244e9592d42581f4d36925e586c Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sat, 12 Jun 2021 18:50:31 +0200
Subject: [PATCH 03/18] Update plot.py

---
 xarray/plot/plot.py | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 94ab90ebda1..8859704ead0 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -925,18 +925,20 @@ def imshow(x, y, z, ax, **kwargs):
             "imshow requires 1D coordinates, try using pcolormesh or contour(f)"
         )
 
+    def _center_pixels(x):
+        if np.issubclass(x.dtype, str):
+            return x[0], x[-1]
+
+        try:
+            xstep = (x[1] - x[0]) / 2.0
+        except IndexError:
+            # Arbitrary default value, similar to matplotlib behaviour
+            xstep = 0.1
+        return x[0] - xstep, x[-1] + xstep
+
     # Centering the pixels- Assumes uniform spacing
-    try:
-        xstep = (x[1] - x[0]) / 2.0
-    except IndexError:
-        # Arbitrary default value, similar to matplotlib behaviour
-        xstep = 0.1
-    try:
-        ystep = (y[1] - y[0]) / 2.0
-    except IndexError:
-        ystep = 0.1
-    left, right = x[0] - xstep, x[-1] + xstep
-    bottom, top = y[-1] + ystep, y[0] - ystep
+    left, right = _center_pixels(x)
+    top, bottom = _center_pixels(y)
 
     defaults = {"origin": "upper", "interpolation": "nearest"}
 

From 21c028fb2fba54ea9bcd95937e28b56b427584e0 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sat, 12 Jun 2021 19:05:03 +0200
Subject: [PATCH 04/18] Update plot.py

---
 xarray/plot/plot.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 8859704ead0..8e610b153be 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -926,7 +926,7 @@ def imshow(x, y, z, ax, **kwargs):
         )
 
     def _center_pixels(x):
-        if np.issubclass(x.dtype, str):
+        if np.issubdtype(x.dtype, str):
             return x[0], x[-1]
 
         try:

From a393e9182791f3fd6253a135717185fda2fd0672 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sat, 12 Jun 2021 19:34:00 +0200
Subject: [PATCH 05/18] Update plot.py

---
 xarray/plot/plot.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 8e610b153be..d889e6c7dd3 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -927,7 +927,7 @@ def imshow(x, y, z, ax, **kwargs):
 
     def _center_pixels(x):
         if np.issubdtype(x.dtype, str):
-            return x[0], x[-1]
+            return None, None
 
         try:
             xstep = (x[1] - x[0]) / 2.0

From eb96ee9717b0b97a4a516bd45d12ab84d5b26e88 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sat, 12 Jun 2021 20:47:09 +0200
Subject: [PATCH 06/18] ismhow and plot_surface doesn't support cats

---
 xarray/plot/plot.py       | 5 +++++
 xarray/tests/test_plot.py | 6 +++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index d889e6c7dd3..b7347d62c37 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -919,6 +919,8 @@ def imshow(x, y, z, ax, **kwargs):
     The pixels are centered on the coordinates. For example, if the coordinate
     value is 3.2, then the pixels for those coordinates will be centered on 3.2.
     """
+    if any(np.issubdtyp(v.dtype, str) for v in [x, y]):
+        raise TypeError("ax.imshow does not support categorical data.")
 
     if x.ndim != 1 or y.ndim != 1:
         raise ValueError(
@@ -1059,5 +1061,8 @@ def surface(x, y, z, ax, **kwargs):
 
     Wraps :py:meth:`matplotlib:mpl_toolkits.mplot3d.axes3d.Axes3D.plot_surface`.
     """
+    if any(np.issubdtyp(v.dtype, str) for v in [x, y]):
+        raise TypeError("ax.plot_surface does not support categorical data.")
+
     primitive = ax.plot_surface(x, y, z, **kwargs)
     return primitive
diff --git a/xarray/tests/test_plot.py b/xarray/tests/test_plot.py
index afbd228b9aa..436446edbf2 100644
--- a/xarray/tests/test_plot.py
+++ b/xarray/tests/test_plot.py
@@ -1163,7 +1163,11 @@ def test_3d_raises_valueerror(self):
 
     def test_nonnumeric_index(self):
         a = DataArray(easy_array((3, 2)), coords=[["a", "b", "c"], ["d", "e"]])
-        self.plotfunc(a)
+        if self.plotfunc.__name__ in ["imshow", "surface"]:
+            with pytest.raises(TypeError, match=r"does not support categorical data"):
+                self.plotfunc(a)
+        else:
+            self.plotfunc(a)
 
     def test_multiindex_raises_typeerror(self):
         a = DataArray(

From f6ff2d21dc4d3c35715443e6d537ba0c69da3579 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sat, 12 Jun 2021 23:31:02 +0200
Subject: [PATCH 07/18] Update plot.py

---
 xarray/plot/plot.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index b7347d62c37..92fb0b61ee7 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -919,7 +919,7 @@ def imshow(x, y, z, ax, **kwargs):
     The pixels are centered on the coordinates. For example, if the coordinate
     value is 3.2, then the pixels for those coordinates will be centered on 3.2.
     """
-    if any(np.issubdtyp(v.dtype, str) for v in [x, y]):
+    if any(np.issubdtype(v.dtype, str) for v in [x, y]):
         raise TypeError("ax.imshow does not support categorical data.")
 
     if x.ndim != 1 or y.ndim != 1:
@@ -1061,7 +1061,7 @@ def surface(x, y, z, ax, **kwargs):
 
     Wraps :py:meth:`matplotlib:mpl_toolkits.mplot3d.axes3d.Axes3D.plot_surface`.
     """
-    if any(np.issubdtyp(v.dtype, str) for v in [x, y]):
+    if any(np.issubdtype(v.dtype, str) for v in [x, y]):
         raise TypeError("ax.plot_surface does not support categorical data.")
 
     primitive = ax.plot_surface(x, y, z, **kwargs)

From bd9f9bafb78a1c15762d7ab1d7e0b1fbe62490d4 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sun, 13 Jun 2021 06:36:50 +0200
Subject: [PATCH 08/18] Update plot.py

---
 xarray/plot/plot.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 92fb0b61ee7..510f0e985e7 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -932,7 +932,7 @@ def _center_pixels(x):
             return None, None
 
         try:
-            xstep = (x[1] - x[0]) / 2.0
+            xstep = 0.5 * (x[1] - x[0])
         except IndexError:
             # Arbitrary default value, similar to matplotlib behaviour
             xstep = 0.1

From c346c2567e92bfb9f571e23a50b236855099f9a1 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Sun, 13 Jun 2021 11:48:37 +0200
Subject: [PATCH 09/18] Better comments

---
 xarray/plot/plot.py       | 13 ++++++++-----
 xarray/tests/test_plot.py |  1 +
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 510f0e985e7..37e9a1c40d9 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -927,20 +927,23 @@ def imshow(x, y, z, ax, **kwargs):
             "imshow requires 1D coordinates, try using pcolormesh or contour(f)"
         )
 
-    def _center_pixels(x):
+    def _maybe_center_pixels(x):
+        """Center the pixels on the coordinates."""
         if np.issubdtype(x.dtype, str):
+            # Categorical data cannot be centered:
             return None, None
 
         try:
+            # Center the pixels assuming uniform spacing:
             xstep = 0.5 * (x[1] - x[0])
         except IndexError:
-            # Arbitrary default value, similar to matplotlib behaviour
+            # Arbitrary default value, similar to matplotlib behaviour:
             xstep = 0.1
         return x[0] - xstep, x[-1] + xstep
 
-    # Centering the pixels- Assumes uniform spacing
-    left, right = _center_pixels(x)
-    top, bottom = _center_pixels(y)
+    # Try to center the pixels:
+    left, right = _maybe_center_pixels(x)
+    top, bottom = _maybe_center_pixels(y)
 
     defaults = {"origin": "upper", "interpolation": "nearest"}
 
diff --git a/xarray/tests/test_plot.py b/xarray/tests/test_plot.py
index 436446edbf2..2252b9ff060 100644
--- a/xarray/tests/test_plot.py
+++ b/xarray/tests/test_plot.py
@@ -1164,6 +1164,7 @@ def test_3d_raises_valueerror(self):
     def test_nonnumeric_index(self):
         a = DataArray(easy_array((3, 2)), coords=[["a", "b", "c"], ["d", "e"]])
         if self.plotfunc.__name__ in ["imshow", "surface"]:
+            # ax.imshow and ax.plot_surface errors with nonnumerics:
             with pytest.raises(TypeError, match=r"does not support categorical data"):
                 self.plotfunc(a)
         else:

From f922ce3c66d65705dbf395e716285ad22d60bf2b Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Mon, 14 Jun 2021 19:45:24 +0200
Subject: [PATCH 10/18] Update whats-new.rst

---
 doc/whats-new.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index ff67ea20073..355e19ee2a8 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -38,6 +38,8 @@ New Features
   By `Thomas Hirtz <https://github.com/thomashirtz>`_.
 - allow passing a function to ``combine_attrs`` (:pull:`4896`).
   By `Justus Magin <https://github.com/keewis>`_.
+- Allow plotting categorical data (:pull:`5464`).
+  By `Jimmy Westling <https://github.com/illviljan>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~

From 3a1c196401915a63733c45d1916af3656b7a71ee Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Wed, 16 Jun 2021 18:19:20 +0200
Subject: [PATCH 11/18] Don't handle pixel centering when inputs are strings.

Has to be refixed if/when ax.imshow allows categorical data.
---
 xarray/plot/plot.py | 29 ++++++++++++-----------------
 1 file changed, 12 insertions(+), 17 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 37e9a1c40d9..e4926395f84 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -927,23 +927,18 @@ def imshow(x, y, z, ax, **kwargs):
             "imshow requires 1D coordinates, try using pcolormesh or contour(f)"
         )
 
-    def _maybe_center_pixels(x):
-        """Center the pixels on the coordinates."""
-        if np.issubdtype(x.dtype, str):
-            # Categorical data cannot be centered:
-            return None, None
-
-        try:
-            # Center the pixels assuming uniform spacing:
-            xstep = 0.5 * (x[1] - x[0])
-        except IndexError:
-            # Arbitrary default value, similar to matplotlib behaviour:
-            xstep = 0.1
-        return x[0] - xstep, x[-1] + xstep
-
-    # Try to center the pixels:
-    left, right = _maybe_center_pixels(x)
-    top, bottom = _maybe_center_pixels(y)
+    # Centering the pixels- Assumes uniform spacing
+    try:
+        xstep = (x[1] - x[0]) / 2.0
+    except IndexError:
+        # Arbitrary default value, similar to matplotlib behaviour
+        xstep = 0.1
+    try:
+        ystep = (y[1] - y[0]) / 2.0
+    except IndexError:
+        ystep = 0.1
+    left, right = x[0] - xstep, x[-1] + xstep
+    bottom, top = y[-1] + ystep, y[0] - ystep
 
     defaults = {"origin": "upper", "interpolation": "nearest"}
 

From c22b3be35c1f78719a4052a57b75192815874037 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Wed, 16 Jun 2021 18:43:42 +0200
Subject: [PATCH 12/18] Revert "Don't handle pixel centering when inputs are
 strings."

This reverts commit 3a1c196401915a63733c45d1916af3656b7a71ee.
---
 xarray/plot/plot.py | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index e4926395f84..37e9a1c40d9 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -927,18 +927,23 @@ def imshow(x, y, z, ax, **kwargs):
             "imshow requires 1D coordinates, try using pcolormesh or contour(f)"
         )
 
-    # Centering the pixels- Assumes uniform spacing
-    try:
-        xstep = (x[1] - x[0]) / 2.0
-    except IndexError:
-        # Arbitrary default value, similar to matplotlib behaviour
-        xstep = 0.1
-    try:
-        ystep = (y[1] - y[0]) / 2.0
-    except IndexError:
-        ystep = 0.1
-    left, right = x[0] - xstep, x[-1] + xstep
-    bottom, top = y[-1] + ystep, y[0] - ystep
+    def _maybe_center_pixels(x):
+        """Center the pixels on the coordinates."""
+        if np.issubdtype(x.dtype, str):
+            # Categorical data cannot be centered:
+            return None, None
+
+        try:
+            # Center the pixels assuming uniform spacing:
+            xstep = 0.5 * (x[1] - x[0])
+        except IndexError:
+            # Arbitrary default value, similar to matplotlib behaviour:
+            xstep = 0.1
+        return x[0] - xstep, x[-1] + xstep
+
+    # Try to center the pixels:
+    left, right = _maybe_center_pixels(x)
+    top, bottom = _maybe_center_pixels(y)
 
     defaults = {"origin": "upper", "interpolation": "nearest"}
 

From a9e382657b5b5a1cbdb8e5bdc093879c73ab3544 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Wed, 16 Jun 2021 18:47:48 +0200
Subject: [PATCH 13/18] Assume matplotlib handles the categoricals.

---
 xarray/plot/plot.py | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index 37e9a1c40d9..db4e1b32a64 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -919,9 +919,6 @@ def imshow(x, y, z, ax, **kwargs):
     The pixels are centered on the coordinates. For example, if the coordinate
     value is 3.2, then the pixels for those coordinates will be centered on 3.2.
     """
-    if any(np.issubdtype(v.dtype, str) for v in [x, y]):
-        raise TypeError("ax.imshow does not support categorical data.")
-
     if x.ndim != 1 or y.ndim != 1:
         raise ValueError(
             "imshow requires 1D coordinates, try using pcolormesh or contour(f)"
@@ -1064,8 +1061,5 @@ def surface(x, y, z, ax, **kwargs):
 
     Wraps :py:meth:`matplotlib:mpl_toolkits.mplot3d.axes3d.Axes3D.plot_surface`.
     """
-    if any(np.issubdtype(v.dtype, str) for v in [x, y]):
-        raise TypeError("ax.plot_surface does not support categorical data.")
-
     primitive = ax.plot_surface(x, y, z, **kwargs)
     return primitive

From e1c45fdcca53b7cc626af3b04634d4827a55a0b7 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Wed, 16 Jun 2021 18:51:54 +0200
Subject: [PATCH 14/18] Update plot.py

---
 xarray/plot/plot.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index db4e1b32a64..e111a503dc9 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -919,6 +919,7 @@ def imshow(x, y, z, ax, **kwargs):
     The pixels are centered on the coordinates. For example, if the coordinate
     value is 3.2, then the pixels for those coordinates will be centered on 3.2.
     """
+
     if x.ndim != 1 or y.ndim != 1:
         raise ValueError(
             "imshow requires 1D coordinates, try using pcolormesh or contour(f)"

From 3e6799bfa10e8a3cf7d35e80212f1dc105995963 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Wed, 16 Jun 2021 23:40:58 +0200
Subject: [PATCH 15/18] imshow has some support for strings.

---
 xarray/plot/plot.py | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/xarray/plot/plot.py b/xarray/plot/plot.py
index e111a503dc9..10ebcc07664 100644
--- a/xarray/plot/plot.py
+++ b/xarray/plot/plot.py
@@ -925,11 +925,13 @@ def imshow(x, y, z, ax, **kwargs):
             "imshow requires 1D coordinates, try using pcolormesh or contour(f)"
         )
 
-    def _maybe_center_pixels(x):
+    def _center_pixels(x):
         """Center the pixels on the coordinates."""
         if np.issubdtype(x.dtype, str):
-            # Categorical data cannot be centered:
-            return None, None
+            # When using strings as inputs imshow converts it to
+            # integers. Choose extent values which puts the indices in
+            # in the center of the pixels:
+            return 0 - 0.5, len(x) - 0.5
 
         try:
             # Center the pixels assuming uniform spacing:
@@ -937,11 +939,12 @@ def _maybe_center_pixels(x):
         except IndexError:
             # Arbitrary default value, similar to matplotlib behaviour:
             xstep = 0.1
+
         return x[0] - xstep, x[-1] + xstep
 
-    # Try to center the pixels:
-    left, right = _maybe_center_pixels(x)
-    top, bottom = _maybe_center_pixels(y)
+    # Center the pixels:
+    left, right = _center_pixels(x)
+    top, bottom = _center_pixels(y)
 
     defaults = {"origin": "upper", "interpolation": "nearest"}
 
@@ -972,6 +975,13 @@ def _maybe_center_pixels(x):
 
     primitive = ax.imshow(z, **defaults)
 
+    # If x or y are strings the ticklabels have been replaced with
+    # integer indices. Replace them back to strings:
+    for axis, v in [("x", x), ("y", y)]:
+        if np.issubdtype(v.dtype, str):
+            getattr(ax, f"set_{axis}ticks")(np.arange(len(v)))
+            getattr(ax, f"set_{axis}ticklabels")(v)
+
     return primitive
 
 

From 1dc00c1f370765417a83eaa9cdd0e505f10251f4 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Wed, 16 Jun 2021 23:42:24 +0200
Subject: [PATCH 16/18] imshow test doesn't need to raise anymore

---
 xarray/tests/test_plot.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/tests/test_plot.py b/xarray/tests/test_plot.py
index 2252b9ff060..c6f5afafd52 100644
--- a/xarray/tests/test_plot.py
+++ b/xarray/tests/test_plot.py
@@ -1163,7 +1163,7 @@ def test_3d_raises_valueerror(self):
 
     def test_nonnumeric_index(self):
         a = DataArray(easy_array((3, 2)), coords=[["a", "b", "c"], ["d", "e"]])
-        if self.plotfunc.__name__ in ["imshow", "surface"]:
+        if self.plotfunc.__name__ == "surface":
             # ax.imshow and ax.plot_surface errors with nonnumerics:
             with pytest.raises(TypeError, match=r"does not support categorical data"):
                 self.plotfunc(a)

From f08f22dfaa959bbe2ec7001e7d88c919426f9318 Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Wed, 16 Jun 2021 23:59:11 +0200
Subject: [PATCH 17/18] catch plot_surface error

---
 xarray/tests/test_plot.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xarray/tests/test_plot.py b/xarray/tests/test_plot.py
index c6f5afafd52..d59d4015385 100644
--- a/xarray/tests/test_plot.py
+++ b/xarray/tests/test_plot.py
@@ -1164,8 +1164,8 @@ def test_3d_raises_valueerror(self):
     def test_nonnumeric_index(self):
         a = DataArray(easy_array((3, 2)), coords=[["a", "b", "c"], ["d", "e"]])
         if self.plotfunc.__name__ == "surface":
-            # ax.imshow and ax.plot_surface errors with nonnumerics:
-            with pytest.raises(TypeError, match=r"does not support categorical data"):
+            # ax.plot_surface errors with nonnumerics:
+            with pytest.raises(np.core._exceptions._UFuncNoLoopError):
                 self.plotfunc(a)
         else:
             self.plotfunc(a)

From 43d669638d19f66703cdeda39bc7968cb6b7965e Mon Sep 17 00:00:00 2001
From: Illviljan <14371165+Illviljan@users.noreply.github.com>
Date: Thu, 17 Jun 2021 20:08:22 +0200
Subject: [PATCH 18/18] Update xarray/tests/test_plot.py

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>
---
 xarray/tests/test_plot.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/tests/test_plot.py b/xarray/tests/test_plot.py
index d59d4015385..e833654138a 100644
--- a/xarray/tests/test_plot.py
+++ b/xarray/tests/test_plot.py
@@ -1165,7 +1165,7 @@ def test_nonnumeric_index(self):
         a = DataArray(easy_array((3, 2)), coords=[["a", "b", "c"], ["d", "e"]])
         if self.plotfunc.__name__ == "surface":
             # ax.plot_surface errors with nonnumerics:
-            with pytest.raises(np.core._exceptions._UFuncNoLoopError):
+            with pytest.raises(Exception):
                 self.plotfunc(a)
         else:
             self.plotfunc(a)